1. Field of the Invention
The field of the invention is the manufacture of cement, more particularly the field is the mixing of raw materials to produce a cement raw mix for feeding into a kiln.
2. Brief Description of the Prior Art
In the cement making process, limestone is mixed with other materials to form a cement raw mix. Typical materials that may be mixed with the limestone to form the raw mix are diaspore (source of aluminum oxide), tripoli (source of silica oxide), and fly ash (source of iron oxide). The raw mix produced is then fed into a kiln where it undergoes a calcination process.
The characteristics of the final product are determined by (1) chemical composition of the raw mix, (2) conditions in the kiln, and (3) the extent of the exposure of the raw mix in the kiln during the calcination process. Further, the extent to which the raw mix is to be exposed in the kiln will vary depending upon the actual composition of the raw mix. In producing a quality cement, the raw mix should ideally have a constant homogenous composition with a small standard of deviation.
However, it is difficult to produce a constant homogeneous mix because the raw materials, particularly the limestone, are susceptible to variations in the chemical composition as they are fed into the raw mix. For example, the composition of the limestone may vary over time because of: (a) variations in the extent of overburden mixed with the limestone; (b) variations within the chemical composition within the rock itself; and (c) changes in the moisture content in the stone.
Other problems encountered in maintaining a uniform composition in the raw mix involve the extremely large quantities of raw material being mixed and the possibility of weighfeeder malfunctions. Because of the extremely large quantities involved, direct analysis of the stone may not even accurately represent the composition of the stone at any one given point in time, much less accurately represent the composition over any extended period. When a weighfeeder malfunction goes undetected, the raw materials may be mixed in entirely different ratios from those intended.
In the past, it has been a common practice for a "mix analyst" to periodically test the raw mix composition. After testing the raw mix composition, the mix analyst then compares the actual composition with the desired composition of the raw mix to be fed into the kiln. Based upon this comparison, the mix analyst adjusts the relative proportions of the raw materials being supplied to the raw mix to compensate for deficiencies in any particular necessary raw material. For instance, after checking the chemical composition of the raw mix at a given time, the analyst may determine that the composition is low in silica oxide. To compensate for this deficiency, the analyst then approximates how much more sand is needed to eliminate the silica oxide deficiency, and adds proportionately more sand over the next time interval. At the end of the next time interval, he agains checks the raw mix composition to determine whether any further correction need to be made. This process has inherent limitations in that is is dependent upon the intuition and experience of the mix analyst and it is not directly responsive to changes in the actual composition of the raw materials. And while the general process can produce satisfactory results where the raw mix is batch mixed or where the raw materials are of unusually uniform composition, it is unsuitable for the newer large capacity plants which utilize a constant kiln feed with reduced retention time in the kiln because of pre-calcination. In these newer plants, composition uniformity is a greater factor as variations in the composition more substantially affect the quality of the final product.
Various automated processes have been developed which compare the actual compositions of the raw mix with a target composition, and adjust the proportions of the raw materials being mixed to compensate for deviations from the target composition. See U.S. Pat. Nos. 3,233,073 to L. E. Veo et al.; 3,835,873 to Wildpaner; and 3,602,488 to Romig. The automated systems which have been developed, however, do not provide for the periodic indirect adjustment of the estimated chemical compositions of the raw materials being mixed; their initiation procedures include direct analysis of the limestone; and the systems do not incorporate weighfeeder malfunction detection in the automated material proportioning aspect of their process.